9

LTC1405

Table 2. Comparison of Analog Input Configurations

SUPPLIES

COUPLING

GAIN

+

–

COMMENTS

±5V

DC

4.096V

1x

±2.048

0

Best SNR, THD

5V

DC

4.096V

2x

2.5

± 1.024

2.5

Best SINAD, THD for Single Supply

5V

DC

2.048V

1x

2.5

± 1.024

2.5

Worse Noise than Above Case

5V

DC

4.096V

1x

2.5

± 2.048

2.5

Best Single Supply Noise, THD Is Not Optimal

5V

DC

4.096V

1x

0 to 4.096

2.048

Same As Above

±5V

AC

4.096V

1x

±1.024

±1.024

Very Best SNR, THD

(Transformer)

5V

AC

4.096V

1x

2.5

± 1.024

2.5

± 1.024

Very Best SNR, THD for Single Supply

(Transformer)

APPLICATIO S I FOR ATIO

1405 F05

1

µF

2.5V

LTC1405

5V

1405 F06

4.096V

0V

5V

2.048V

LTC1405

5V

SENSE

Figure 4. DC Coupling a Ground

Centered Signal (Dual Supply System)

Figure 5. DC Coupling a Signal Centered

Around 2.5V (Single Supply System)

Figure 6. DC Coupling a 0V to 4.096V Signal

AC Coupling the Input

The analog inputs to the LTC1405 can also be AC coupled

through a capacitor, though in most cases it is simpler to

directly couple the input to the ADC. Figure 7 shows an

example where the input signal is centered around ground

and the ADC operates from a single 5V supply. Note that

the performance would improve if the ADC was operated

from a dual supply and the input was directly coupled (as

in Figure 4). With AC coupling the DC resistance to ground

offset accuracy.

DC Coupling the Input

In most applications the analog input signal can be directly

coupled to the LTC1405 inputs. If the input signal is

centered around ground, such as when dual supply op

system with the input signal centered around 2.5V, con-

signal is not centered around ground or 2.5V, the voltage

or a voltage reference (Figure 6).

1405 F07

1

µF

0V

C

C

RR

LTC1405

5V

Figure 7. AC Coupling to the LTC1405. Note That the Input Signal

Can Almost Always Be Directly Coupled with Better Performance

1405 F04

1

µF

0V

LTC1405

5V

–5V